Masks used in X-Ray lithography must, of necessity be very thin and flat. The masks must be in the order of 0.5 to 10 microns or less in thickness to transmit the X-Ray beam in sufficient strength to adequately expose the photoresist coating of a wafer. The masks must be optically flat, e.g., ideally to half a wavelength or better to minimize overlay registration error. The present invention provides a method for providing such an optically flat X-Ray mask.